Piazza Giordano Bruno is dominated by the imposing structure of the Chiesa di San Domenico.
During the early Middle Ages, this area was the site of the local horse market and fair. Tradition has it that the church was designed by Giovanni Pisano. In any case, its construction was completed in 1458, the year in which is was consecrated to St. Stephen by Pope Pius II. Before the 17th-century renovation of the interior, the building presented an enormous ‘hallenkirche’ plan with octagonal pilasters in laterite stone, pointed arches and large stained-glass windows.
The bell-tower was once surmounted by a high steeple dating from the late 1400s and was the work of Gasperino di Antonio of Lombardy. It was shortened during the 15th century, probably for reasons of instability, although tradition identifies in this act the same motivations which prompted Pope Paul III to demolish the upper level of the church of San Ercolano. The front staircase was designed by Girolamo Ciofi of Cortona (1640).
The doorway is dated 1596. Above its remain traces of a rose-window by Benedetto di Valdorcia of Siena (1415).

Next to the church, walk along the narrow Via del Castellano for a terrific view of the south aisle chapel walls of the church. The rhythm of the red and white stone work is interrupted by a large gothic mullioned-window; the upper region bears a dentil shaped ornamentation also in red and white stone. Nearby, the house next to the Auditorium Marianum contains an early 14th-century painting of the Madonna and Child.